var cards = $('.cards'); // only fetches previous cards, not the new ones

my_function( cards );

def fetch_cards
    respond_to do |format|
        format.js
    end
end

$.ajax('your/resources.json',
    complete: function(jqXHR, status){
      // this will be executed once the server has completed your request
      my_function(cards);
    }
);